The present invention relates to a process for reducing the level of metal ions in film forming novolak resins, photoresist compositions, and other raw materials for photoresist compositions such as solvents, additives, dyes, and surfactants. The present invention also relates to a process that utilizes a specially designed filter element, hereinafter referred to as the "Ion Exchange Pack", as shown in FIG. 1 below. The Ion Exchange Pack is loaded with either a cation exchange resin or an anion exchange resin, a mixture of both, or a chelating ion exchange resin. The resulting loaded Ion Exchange Pack is especially efficient for reducing metal ions and other contaminants from products such as novolak resins, photoresist compositions, and other raw materials for photoresist compositions. The metal ions, cations and anions, are removed by an adsorption process utilizing a cation or anion exchange resin, respectively, or a combination of the two, in such an Ion Exchange Pack.
Photoresist compositions are used in microlithography processes for producing electronic components such as computer hard drives, semiconductor chips and integrated circuits. Generally, in these processes, a thin film coating of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to substantially evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure to radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate.
Metal ion contamination has been a problem for a long time in the fabrication of high density integrated circuits, computer hard drives and computer chips, often leading to increased defects, yield losses, degradation and decreased performance. In plasma processes, metal ions such as sodium and iron, when they are present in a photoresist, can cause contamination especially during plasma stripping. However, these problems can be overcome to a substantial extent during the fabrication process, for example, by utilizing HCl gettering of the contaminants during a high temperature anneal cycle.
As electronic devices have become more sophisticated, these problems have become much more difficult to overcome. When silicon wafers are coated with a liquid positive photoresist and subsequently stripped off, such as with oxygen microwave plasma, the performance and stability of the semiconductor device is often seen to decrease because of the presence of what would be considered very low levels of metal ions. As the plasma stripping process is repeated, more degradation of the device frequently occurs. A primary cause of such problems has been found to be metal ion contamination in the photoresist, particularly sodium and iron ions. Metal ion levels of than 100 ppb (parts per billion in the photoresist have sometimes been found to adversely affect the properties of such electronic devices.
Film forming novolak resins are frequently used a polymeric binder in liquid photoresist formulations. These resins are typically produced by conducting a condensation reaction between formaldehyde and one or more multi-substituted phenols, in the presence of an acid catalyst, such as oxalic acid or maleic anhydride. In producing sophisticated semiconductor devices, it has become increasingly important to provide novolak resins having metal ion contamination levels below 50 ppb each.
There are two types of photoresist compositions, negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become less soluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to such a solution. Thus, treatment of an exposed negative-working resist with a developer causes removal of the non-exposed areas of the photoresist coating and the creation of a negative image in the coating. Thereby uncovering a desired portion of the underlying substrate surface on which the photo-resist composition was deposited. On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the photoresist composition exposed to the radiation become more soluble to the developer solution (e.g. a rearrangement reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working photoresist with the developer causes removal of the exposed areas of the coating and the creation of a positive image in the photoresist coating. Again, a desired portion of the underlying substrate surface is uncovered.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gases and the like. The etchant solution or plasma gases etch that portion of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected and, thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface. In some instances, it is desirable to heat treat the remaining photoresist layer, after the development step and before the etching step, to increase its adhesion to the underlying substrate and its resistance to etching solutions.
Positive working photoresist compositions are currently favored over negative working resists because the former generally have better resolution capabilities and pattern transfer characteristics. Photoresist resolution is defined as the smallest feature which the resist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many manufacturing applications today, resist resolution on the order of less than one micron are necessary. In addition, it is almost always desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate.